This invention relates to transmit/receive (T/R) apparatus of an electrically steered phased array antenna and in particular to a transmit/receive switch for protecting receiver apparatus when operating in a transmit mode.
Small size and light weight are key features of a T/R module comprising transmit, receive and digital controller circuits for airborne and space-based phased array antenna applications. An L-band T/R module measuring 1".times.2" and weighing 16 grams is feasible today using gallium arsenide (GaAs) monolithic microwave integrated circuits (MMIC). Such a T/R module is described in a paper entitled, "Lightweight L-Band T/R Modules for SBR Applications," by Peter Maloney and John Sasonoff, Government Microcircuit Applications Conference, November 1991, pp. 191-194. The receive portion of the T/R module comprising a T/R switch, an impedance matching and bias feed network, and a low noise amplifier (LNA) must provide very low noise figures (high sensitivity) minimal power consumption, and minimal distortion of an RF input signal. When GaAs MMIC technology is used in the transmit, receive, and digital controller portions of the T/R module, the design of the circuits of the receive portion such as the T/R switch and impedance matching and bias feed network often results in undesirable compromises due to the conflicting constraints of low noise (low-loss), the impedance transformations required, and high power handling capability in an OFF state.
The T/R switch is used to provide a matched termination load to the RF input source such as a circulator during a transmit mode of operation and to perform as an extremely low loss transparent switch during a receive mode of operation to preserve a minimum noise figure of the low noise amplifier. In the prior art a well known T/R switch comprises a .pi. network having 1/4 wavelength lines (or L/C equivalents) in each leg of the .pi. along with field effect transistor (FET) switches. However, for space-based or airborne applications where small size and light weight are key requirements, the prior art techniques are not satisfactory.
An impedance matching and bias feed network coupled between the TR switch and the LNA performs the task of impedance transformation such as transforming a 50 ohm RF source input to a higher impedance such as the value of the input impedance of the low noise amplifier. Another function of such a network at the input to the low noise amplifier is to provide a bias voltage to an input circuit of the LNA without degrading the LNA performance characteristics. One type of matching network well known in the art comprises an L-C series network with a resistor shunted to a gate bias voltage. The impedance transformation takes place in the capacitor while the inductor serves to tune the reactive part of the FET input impedance. However, the drawbacks of this network include the size of the coil being relatively large and the gate bias resistor shunting an FET gate in the low noise amplifier which results in losses that raise the noise figure.